1. Field of the Invention
This invention relates to surface acoustic wave (SAW) pressure sensors, and more particularly to vacuum encapsulating structures therefor.
2. Description of the Prior Art
SAW pressure sensors are well known in the art, as reported in U.S. Pat. Nos. 3,978,731 and 4,100,811. Briefly stated, SAW delay lines which include a planar substrate having two major surfaces with electro-acoustic transducers disposed in an active signal region on one of the surfaces are adapted to provide SAW pressure sensors by forming a flexible, deformable diaphragm in the active signal region. The diaphragm is formed between the surface of the substrate which includes the active signal region and a parallel surface provided by the end wall of an interior cylindrical cavity, or bore, formed in the second major surface. The cavity acts as a fluid conduit to the interior surface of the diaphragm for applied pressure signals which apply stress to the diaphragm causing it to deform and change the acoustic wave propagation characteristics in the active signal region of the substrate. By connecting the SAW delay line to an external oscillator the change in acoustic wave propagation velocity may be measured as a change in the frequency of oscillation, all of which is disclosed in the hereinbefore referenced patents.
When used as absolute pressure sensing devices, the SAW pressure sensors must be vacuum encapsulated to provide zero psi on a reference surface of the diaphragm (the active signal region surface) while permitting access to the opposite surface of the diaphragm (the interior surface formed by the cavity end wall) for the sensed pressure signals. The encapsulating structure must also permit external electrical connection to the transducers of the delay line and, ideally, must not induce thermal strain in the SAW active signal region resulting from temperature cycling of the structure over the operating temperature range of the sensor. The requirement to prevent, or minimize induced thermal strain presents difficulties when there are different temperature coefficients of expansion between the SAW substrate material and the vacuum encapsulating material. The problem is particularly acute when the SAW substrate itself comprises piezoelectric material, such as quartz which has anisotropic temperature coefficients of expansion. One structure which satisfies all of the requirements, especially that of minimizing induced strain, is described in a commonly owned, copending application of the same assignee entitled VACUUM ENCAPSULATION FOR SURFACE ACOUSTIC WAVE (SAW) DEVICES, U.S. Ser. No. 945,359, filed on Sept. 25, 1978 by D. E. Cullen and R. A. Wagner, wherein the vacuum structure is formed from the same crystal material comprising the substrate, which results in identical expansion characteristics over temperature and which is electrically insulative permitting a bond of the structure directly across the transducer conductors. As a result, the active signal region is maintained in a vacuum while the opposite surface of the diaphragm is readily accessible to the sensed pressure signals. There are many instances, however, where a metal vacuum structure would be preferred due to the operating environment. While suitable metal packaging techniques are available for providing the electrical interconnection to the transducers, the combination of the dissimilar materials, i.e. metal and crystal, results not only in induced strain in the SAW sensor diaphragm but, for piezoelectric substrates with anisotropic temperature characteristics, the strain may become so severe as to cause the rupture of the vacuum seal of the structure to the substrate. At the present time this provides a definite limitation on both the accuracy and the maximum operating temperature range of metal encapsulated SAW pressure sensors.